Epitaxial Multilayers of (Sr,Ba)Nb 2 O 6 and Conducting Films on (001) Mgo Substrates
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ABSTRACT (Sr,Ba)Nb 2 0 6 (SBN) is a very promising material for nonlinear optical applications because of its high electro-optic and nonlinear optical coefficients. For these applications, SBN requires ferroelectric poling along the optical axis. Conducting layers such as Pt or YBCO must therefore be deposited to provide electrodes above and below the SBN film. We have investigated the epitaxial growth of multilayers of Sr 0 .61Ba 0 .39 Nb 2 O 6 /Pt and Sr 0 .61 Ba 0 .39 Nb 2O 6 /YBCO thin films on (001) MgO substrates by pulsed laser deposition. X-ray diffraction 20 scans indicate epitaxial growth of SBN/Pt/MgO and SBN/YBCO/MgO heterostructures with their c-axes perpendicular to the substrate plane. X-ray phi scans indicate single crystal Pt and YBCO growth with one in plane orientation. Atomic Force Microscopy shows surface roughness of 2.19 nm, and no evidence of particles is observed.
INTRODUCTION Strontium barium niobate (SBN) possesses extremely high electro-optical coefficients (r33 = 1340 pm/V)1 which makes it desirable for electro-optic modulation and beam steering applications in integrated optics. It also possesses large nonlinear optical coefficients (d 33 = 12.8 pm/V) 2 making it an attractive material for applications in nonlinear optics such as laser frequency conversion. SBN is a ferroelectric material at room temperature and in its as-grown state it is presumed to consist of randomly oriented domains. These random domains will not produce an average electro-optic or nonlinear optic effect and for the applications described above the film must therefore be poled. The ability to control the spontaneous polarization direction is also required in order to periodically pole the films for quasi phase matching in optical frequency conversion. Ferroelectric poling requires the application of an electric field greater than the coercive field (9 KV/cm for SBN) along the optical axis (c axis for SBN). The desired SBN orientation for waveguide applications is with its optical axis orthogonal to the plane of the interface, so that the largest electro-optic and nonlinear optic coefficients can be accessed. This requires the electrodes to be deposited above and below the SBN film. Since the MgO substrate is much thicker and a better insulator than SBN, the bottom electrode must be deposited between the substrate and the SBN film. Otherwise, electrical breakdown will occur before the coercive field strength is reached for the SBN. The SBN layer will be epitaxially grown on top of the electrode layer and we therefore require a good epitaxial match between the SBN film (tetragonal, a=b=3.935 A, c= 12.46 A), electrode material and MgO substrate (cubic, 4.2 A). Two candidates have been investigated: platinum 79 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
which is cubic with a lattice constant of 3.92 A, and yttrium barium copper oxide (YBCO), which is orthorhombic with lattice constants a=3.886 A, b=3.819 A and c=l 1.68 A. EXPERIMENTAL SBN/Pt/MgO and SBN/YBCO/MgO multilayer structures have
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